So yet another UK government official didn't get the memo – the one that says when you're going to a high-level, top secret meeting in Downing Street, try not to arrive carrying papers that can be snapped by waiting photographers. As deputy national security adviser Hugh Powell is the latest to discover, in the age of the zoom lens, they can be read easily. (It turns out there's a freelance photographer who hangs around outside No 10, one Steve Back, who specialises in just such pictures.)

Still, we should be grateful to the unguarded Mr Powell. If he had popped his documents in a folder we would never know that, whatever other action the European Union has in mind on Ukraine, the British government is adamant that the City of London will be exempt. Or as the official text put it, Britain will "not support, for now, trade sanctions … or close London's financial centre to Russians." In other words, even if Russia is in the process of invading a sovereign state, Britain will still do nothing that might dent the profits of the money men in the City.

I say we would never know – but we could have taken a guess. For there is a rather consistent pattern here. In January, George Osborne set out one of the key demands Britain will be making of the EU in the lead-up to the planned in-or-out referendum of 2017, one of those existential needs that must be met if Britain is to stay inside. What was it? "Cast-iron legal protections for the City of London." The chancellor warned that the UK would leave the EU unless the Lisbon Treaty was changed to prevent the imposition of financial services legislation that might rein in the City.

And who can forget the heroic sight of Osborne heading to Brussels exactly a year ago to fight the good fight – hoping to stop our European partners from capping bankers' bonuses? If there is so much as a hint of a challenge to the City, Osborne is ready to pull on his armour and ward off all enemies. No matter if that means promising business-as-usual to Russia, whether or not it has invaded one of its neighbours, it's the City that must come first.

It's quite clear that with 60% of the EU's financial services industry located in the UK, the bankers and fund managers are seen as more than just big business. The British government regards their prosperity as a matter of national security, their interests to be weighed against other geopolitical considerations such as our membership of the EU and the principle that the violation of the borders of sovereign states demands punishment.

Not that Britain is alone in this. Germany is reluctant to come down hard on Russia, its No 1 trading partner, on whom it relies for its domestic energy. France enjoys a lucrative relationship with Moscow, too, and is contracted to build two valuable Mistral-class warships for the Russian navy. They're looking out for their "strategic" industries, just like Osborne.

The result is that Vladimir Putin knows he can scarcely be touched. That would be true of any state that is a nuclear-armed, permanent member of the UN Security Council. But it's truer still of a state whose money and resources the west needs. It makes Russia doubly untouchable.

The lesson for any regime watching the Ukraine crisis unfold is clear. Sending troops into the territory of a sovereign state is completely unacceptable and you will be pushed back – unless, that is, you are strong and hurting you hurts us. Then different rules apply.